New monitor type: Calendar Date Reminder

[fjc0k]

⚠️ Please verify that this feature request has NOT been suggested before.

• I checked and didn't find similar feature request

🏷️ Feature Request Type

New Monitor

🔖 Feature description

This is a generic expiration reminder monitor. Users can set the expiration time and the advance time parameters themselves. Once the reminder time range is reached, a notification will be sent.

✔️ Solution

This monitor offers flexibility and practicality, making it applicable to various scenarios that require expiration reminders but where the expiration dates cannot be automatically obtained. Examples include SaaS service expirations, software license expirations, domain name expirations, and more. Users can customize the reminder parameters according to their needs, ensuring that important expiration dates are not missed.

❓ Alternatives

No response

📝 Additional Context

No response

[CommanderStorm]

• I kind of disagree that this would make a good monitor:
  monitors are supposed to monitor something and not act as just a calendar reminder
  ⇒ Check if it is alive by a machine.
  Your monitor offloads this part to a human, which is likely not desirable.
• Domain expiration is tracked in Domain Expiry Monitor #944.
  For the software expiration you either get an email anyway that this is expiring, this is on autorenewal or you track this in your accounting software

If you disagree, you can still contribute such a monitor and see if Louis accepts/recects it.
Here is our contribution guide: https://github.com/louislam/uptime-kuma/blob/master/CONTRIBUTING.md

An example-pr that implements a monitor according to our new syntax (not just piling anything into monitor.js) can be found here: #3178 or #3308